Field of the Invention
The present invention relates to a polymer nanofiber sheet and a method for producing the polymer nanofiber sheet.
Description of the Related Art
In recent years, polymer nanofiber structures typified by polymer nanofiber sheets have been drawing attention as materials having large specific surface areas. In such structure, nanofibers having a polymer are accumulated and thus are three dimensionally entangled with each other.
However, in the conventional polymer nanofiber structures formed from three dimensionally entangled polymer nanofibers, the fibers are simply, physically entangled with each other. Such a structure has low mechanical strength, is likely to be damaged by tensile force or friction, and is insufficient for practical applications. To address this problem, techniques for improving the mechanical strength of the polymer nanofiber structures have been developed. Japanese Patent Application Laid-Open No. 2011-214170 discloses a technique of performing a partial bonding treatment. In this technique, a thread-shaped polymer nanofiber structure formed by twisting a plurality of polymer nanofibers are heated, and thus the polymer nanofibers are partially bonded to each other. According to Japanese Patent Application Laid-Open No. 2011-214170, the partial bonding treatment enables production of a polymer nanofiber structure having higher strength. Separately, Japanese Patent Application Laid-Open No. 2010-84252 discloses a technique for improving the strength of a laminate including polymer nanofibers by joining at least some of the polymer nanofibers included in the laminate through a crosslinkable substance and discloses a waterproof moisture-permeable laminate produced by this technique.
However, for the technique of Japanese Patent Application Laid-Open No. 2011-214170, it is difficult to control the temperature suitable for partial bonding of polymer nanofibers, for example. Depending on conditions, nanofibers are greatly melted to form a structure that includes fibers having a diameter of several micrometers or more, in some cases. Accordingly, the resulting nanofiber structure itself may have a lower specific surface area. For the technique of Japanese Patent Application Laid-Open No. 2010-84252, such a problem that a crosslinkable substance is detached from a fiber base material during a crosslinking treatment arises depending on the correlation between the fiber base material used and the crosslinkable substance. In addition, depending on the crosslinkable substance, a structure having sufficient strength may not be obtained in some cases.